<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
	<script type="text/javascript">

		alert(a); // 弹出undefined

		//alert(c); 报错，c没有声明

		var a = 123;


		// 弹出 hello!

		myalert();


		function myalert(){

			alert('hello!');

		}


	</script>
</head>
<body>
	
</body>
</html>